The invention relates to a coating composition, a method of applying the same, and a coated substrate comprising the same.
Coatings used for painting motor vehicles and repairing the original paint are required to have good physical properties such as hardness, mechanical strength, and resistance to water, acids, and solvents. The coatings are also required to have good appearance properties, which means that films must be smooth and have a high gloss and high distinctness of image. It is also desirable that all properties are retained under prolonged outdoor weathering.
A large number of cars and transport vehicles are coated with a multilayer topcoat system wherein an unpigmented clearcoat is applied over a pigmented basecoat. Both solvent borne and water borne clearcoats and basecoats are in use. So-called metallic basecoats comprise metallic flakes.
For environmental reasons, it is required to use a coating composition which can be applied easily using spray application at a low volatile organic content (VOC). Coatings with a lower organic solvent content emit lower levels of solvent when they are used and so the atmosphere becomes less polluted.
One way to achieve a lower solvent content is to use so-called high-solids compositions. Such compositions comprise a relatively high level of non-volatile materials such as film forming polymer, pigments, and fillers, and a relatively low level of organic solvent. A problem when formulating high-solids coating compositions is that such compositions have an unacceptably high viscosity due to the high molecular weight of the conventional film forming polymer. The high viscosity gives rise to problems in spray application with poor paint atomization and poor flow-out and, consequently, low gloss levels and poor appearance.
The use of low-molecular weight film forming polymers, which results in adequate application viscosities, has as a disadvantage that the resulting coating is soft and marks easily. The hardness build-up of the coating is therefore unacceptable. Furthermore, when used in a clearcoat composition, due to the solvency of the low-molecular weight film forming polymers, basecoat properties will suffer from strike-in effect, i.e. discolouration of the basecoat due to its salvation by the clearcoat composition.
Low VOC coating compositions are disclosed in EP-A-0 676 431. In this patent application 1,4-cyclohexane dimethanol is mentioned as a component in a coating composition with a VOC of less than 500 g/l. Unfortunately, coating compositions comprising 1,4-cyclohexane dimethanol appear to have an unfavourable pot life/drying balance. Additionally, when the coating composition disclosed in EP-A-0 676 431 is applied as a clearcoat on a basecoat, the basecoat properties will suffer from the above-mentioned strike-in effect.
Japanese patent application JP 59784-96 discloses a resin composition comprising an acrylic polymer, a polyisocyanate, and a diol. These acrylic polymers have a hydroxy value of 34 and 39. Accordingly, the resulting coating composition will be very flexible and non-resistant to water, acids, and solvents. Thus, the disclosed resin composition is unsuitable as a coating composition for car (re)finish applications.
Accordingly, there is a need for a coating composition which combines all the required properties, such as good thinnability, low VOC, good mixing properties, and low application viscosities, and results in a coating with fast drying times at low temperatures, high film hardness, easy polishability, good resistance to water, acids, and solvents, and excellent durability. When used as a clearcoat, properties such as a low strike-in effect to the basecoat and good transparency are also required.